Class-AB amplifiers are used in power-sensitive applications where the processed analog signals present a large crest factor (i.e. large ratio between average and peak instantaneous values) and/or large peak currents are required. One example of such an application is a line driver. Referring now to FIG. 1, there is shown a high level diagram (100) of a conventional class-AB amplifier used as a line driver. In FIG. 1, a voltage amplifier (102) is connected in a feedback circuit and applies an input signal VSIG (104) to a load resistor, RL (106), supplying all the needed load current while isolating the signal source VSIG (104) from the load (106). As is known in the art, for a high open-loop gain in the amplifier (102), the voltage applied on the load resistor (106) will be approximately equal with VSIG (104), the needed large current being supplied by the amplifier.
As opposed to class-A amplifiers, class-AB amplifiers have biasing currents that are signal-dependent in such a way that, when required by a large signal peak, they can source or sink tens of times more current than their quiescent bias currents. One example of class-AB amplifiers well known to those skilled in the art is the two-stage, Miller-compensated designs with a feed-forward-biased class-AB output stage (OS) and a class-A input stage (IS). See Johan H. Huijsing, “Operational Amplifiers: Theory and Design,” Kluwer Academic Publishers, Boston 2001. These amplifiers have large low-frequency gain, small output impedance and a unity-gain bandwidth that is not signal dependent since the input stage transconductance is produced by a class-A current. With an output stage consuming low power, the power dissipated by the class-A stages in front of the output stage becomes important.
Referring now to FIG. 2, there is shown a high level diagram of a conventional feed-forward class-AB amplifier (200). In general, the class-AB amplifier (200) comprises an input stage (202) and an output stage (204) coupled together by a connecting stage (206), also referred to as a class-AB biasing mesh. The class-AB biasing mesh (206) provides the class-AB biasing to the output stage (204) and shifts internal signal phases in the correct way to fully exploit the gain of the class-AB output stage (204). As is known to those skilled in the art, the class-AB biasing mesh (206) does not provide active gain, so the power it burns does not increase the open-loop gain at all frequencies, and thereby reduces power efficiency.
Referring now to FIG. 3, there is shown a schematic diagram (300) of a conventional feed-forward class-AB amplifier which implements a folded-cascode. The amplifier in FIG. 3 comprises an input stage (302) and an output stage (304) coupled to a class-AB biasing mesh (306). In this example, the class-AB biasing mesh (306) is coupled to the input stage (302) through an intermediate stage designed as a folded-cascode (308). The intermediate stage (308) improves the low-frequency gain of the amplifier due to its cascodes and supplies the class-AB biasing mesh (306) with the in-phase current signals needed to drive the gates of the NMOS (310) and PMOS (312) transistors of the output stage (304). As is known to those skilled in the art, the class-AB biasing works by closing two translinear loops by means of biasing the “pm” (314) and “nm” (316) nodes with two voltages generated as the sum of the two gate-source voltages. In quiescent state, the two drain currents in the mesh transistors are designed to be equal and the translinear loops bias the output stage (304) to its quiescent current. When the input stage (302) is tilted, the signal currents drawn into or from the two ends of the mesh are equal and in phase so the total current in the two mesh transistors does not change. The mesh is tilted as well, so the ratio of the two currents in the NMOS and the PMOS mesh transistors is changed. This makes the output stage (304) tilt by either sinking or sourcing current. For example, if the current in the output NMOS transistor increases, then the current in the PMOS transistor decreases. At the limit, the PMOS transistor still drives a minimum current guaranteed by design, so the amplifier recovers very quickly from the fully-tilted state.
For a large-bandwidth amplifier, the intermediate folded-cascode stage only plays a role in supplying the class-AB biasing mesh with the two needed in-phase signal currents. The power consumption in this intermediate stage is at least two times the power of the input stage for slew-rate and linearity reasons. This amounts to a significant power wasted in the intermediate stage without actual improvement of amplifier performance.
Therefore, what is needed is a class-AB amplifier which eliminates this intermediate stage and does not waste power or current between the input stage and the class-AB biasing mesh, thereby increasing the power efficiency of the amplifier.